A roller cutter drill bit is normally formed of three elongated 120.degree. steel sections or legs having essentially the same geometry. The legs are welded together to form a completed drill bit body. A roller cone is rotatably attached to a cantilevered journal formed at the lower end of each leg to complete the drill bit. Heretofore, it has been common practice to use up to eight different machines, eight special holding fixtures and eight set-ups to do all the machining processes to complete one drill bit leg. When using many different machines and set-ups, the reference datum changes with each operation making it extremely difficult and costly to hold cumulative machining tolerances to acceptable limits on a completed bit leg. The following is a typical description of state of the art operations necessary to convert a rough bit leg forging into a finished roller bit leg.
(Operation 1. Commonly termed 120.degree. milling, this step entails clamping the leg forging in a special fixture mounted on a specially constructed milling machine. Flats are then milled 120.degree. apart on the sides of the forging. They are milled so that three legs positioned with the 120.degree. flats abutting form a bit body. On this same set up, identically located dowel holes are formed into the 120.degree. flat surfaces of the leg so that steel dowel pins placed in the holes position the legs to be welded together in a later operation. PA1 Operation 3. This operation consists of rough turning the bit leg bearing journal with the partially machined forging mounted on a special counter balanced fixture in a lathe using the 120.degree. flats as reference datum. The journal is turned leaving stock to be ground to finished specifications in a later operation. PA1 Operation 4. This step involves mounting the partially machined leg forging in a special fixture on a five-axis CNC machining center using the 120.degree. flats and rough turned journal as dual reference datums. The following operations are sequentially performed using one set-up in the machine. PA1 Operation 5. This operation entails hard facing the load side of the journal bearing in the welding station. It may be done manually or by a special robot. PA1 Operation 6. This operation is a manual operation in the weld shop. It entails hard facing of the turned outer surface of the leg, filling dowel holes and reservoir slots. PA1 Operation 7. This operation entails heat treatment of the entire leg. PA1 Operation 8. This operation is rough grinding of the journal using a plunge grinder with the journal bearing ball race as reference datum in a special holding fixture. PA1 Operation 9. This operation is finish grinding the journal using a bucket grinder with the rough ground journal bearing ball race surface used for reference datum. PA1 Operation 10. The operation entails re-milling of the 120.degree. flat surfaces using the finish ground journal ball race surface as reference datum using a special fixture on a 120.degree. mill. The 120.degree. surface must be re-milled because they become distorted during heat treatment of the leg. PA1 Operation 11. This operation entails mounting the bit leg in a special fixture on a 4-axis CNC machining center using both the re-milled 120.degree. surfaces and the ball race of the finish ground journal as dual reference datums. The following operations are sequentially performed using one set-up in the machine.
Operation 2. Commonly termed back turning, entails clamping the leg forging in a special fixture using the previously milled 120.degree. flats as datum. The forging and fixture are mounted on a specially designed bit leg outside diameter turning machine, such as a modified lathe to turn this outside diameter surface.
Step 4a. The lubricant reservoir hole is drilled into the upper tapered shoulder of the leg. PA2 Step 4b. The jet nozzle hole is drilled into the nozzle socket boss on the lower portion of the leg adjacent the bearing journal. PA2 Step 4c. The hole for inserting ball bearings to rotatively affix the roller cutter to the journal is drilled through the lower end of the leg into the ball race on the journal. PA2 Step 4d. The long lubricant hole is drilled from the grease reservoir into the ball insertion hole to supply the bearing with lubricant from the lubricant reservoir. PA2 Step 4e. The lower end of the turned O.D. or shirttail is milled off concentric to the seal backface surface. PA2 Step 4f. The side lubricant pilot hole for the reservoir grease filling operation is drilled adjacent the lubrication reservoir and intersecting the long lube hole of step 4d. PA2 Step 4g. Lubrication retention flats are milled on the journal. PA2 Step 4h. Lubrication access holes are drilled through the journal flat surfaces to intersect the ball insertion hole. PA2 Step 11a. The side lubrication hole (reference Operation 4- Step 4f) is drilled, counter-drilled and tapped. PA2 Step 11b. The shale burn plug hole, located on the journal back face, is drilled, counter drilled and milled. PA2 Step 11c. The pin end is milled off to a critical length and a small reference dowel hole is drilled in the pin end face. PA2 Step 11d. For subsequent bit assembly means, a shallow tapered hole (dimple) is drilled into the leg back face surface.
As is shown, using this many machines, fixtures and set-ups makes holding the necessary tight dimensional tolerances very difficult as the reference datum on the leg shifts with each set-up. This mandates much closer than normal dimensional tolerances on each metal removal operation, making the overall manufacturing time and cost of a individual bit leg inordinately great. This also leads to an undue level of re-work parts or scrap.
Another disadvantage of current manufacturing systems is when any change is made to a bit leg design, for example, a change in the vertical journal angle or the radial off-set of the journal, a new costly set of machining fixtures must be made to accommodate the change.
The present invention overcomes the shortcomings of the current manufacturing of drill bit legs by providing a method of making a bit leg that is extremely more accurate and versatile. Whereas current methods entail using a multitude of different machines, fixtures and set-ups to machine a bit leg, this invention teaches a method to completely machine a bit leg from a forging using only two machines, two holding fixtures and three set-ups of the workpiece. Because the same reference datum is used for all but the first machining operation, the cumulative error is very slight enabling very close dimensional tolerances on the finished leg to be maintained. Because bit legs made by the teachings of this patent require so few machines and set-ups, as compared to present methods, great savings of time and costs are realized. Because the present invention system is so versatile, it is now feasible, from a cost stand point, to accept rush orders for the manufacture of minimal quantifies of standard bits to fill a customer's need.